Blocking highs

Weather Eye
with John Maunder

Some of the answers to the complexities of the climate system are given in my recently published book Fifteen shades of climate... the fall of the weather dice and the butterfly effect. The following are extracts are from pages 96-98..

Blocking patterns occur when centres of high pressure and/or low pressure set up over a region in such a way that they prevent other weather systems from moving through.

When the blocking pattern is in place other systems are forced to go around it. “Blocks” are important features well recognised by weather forecasters and early in a weather forecaster’s education it is learnt that blocking systems steer the fronts and the lows in the usual chaotic atmospheric circulation.

When an upper-level high-or low-pressure system becomes stuck in place due to a lack of steering currents, it is known as being “cut off ”. The usual pattern which leads to this is the jet stream retreating poleward, leaving the then cut-off system behind. Whether or not the system is of high- or low-pressure variety dictates the weather that the block causes. Precisely this situation occurred over the southern United States during late spring and early summer of 2007. A cut-off-low system hovering over the region brought unusually cool temperatures and an extraordinary amount of rain to Texas and Oklahoma, and a cut-off-high near the coast of Georgia caused a drought in the Southeast that same year.

If the block is a high, it will usually lead to dry, warm weather as the air beneath it is compressed and warmed, as happened in southeastern Australia in 1967 and 2006 with resultant extreme droughts. Rainy, cooler weather results if the block is a low.

The seasonal cycle of blocking

A review paper in the Monthly Weather Review December 2013 by Pool, Risby and McIntosh said that the seasonal cycle of blocking in the Australian region was shown to be associated with major seasonal temperature changes over continental Antarctica and Australia and with minor changes over the surrounding oceans. These changes are superimposed on a favourable background state for blocking in the region resulting from a conjunction of physical influences. These include the geographical configuration and topography of the Australian and Antarctic continents and the positive west to east gradient of sea surface temperature in the Indo-Australian sector of the Southern Ocean. Blocking is represented by a blocking index (BI) developed by the Australian Bureau of Meteorology.

The BI has a marked seasonal cycle that reflects seasonal changes in the strength of the westerly winds in the mid-troposphere at selected latitudes. Significant correlations between the BI at Australian longitudes and rainfall have been demonstrated in southern and central Australia for the austral autumn, winter, and spring. Patchy positive correlations are evident in the south during summer but significant negative correlations are apparent in the central tropical north. By decomposing the rainfall into its contributions from identifiable synoptic types during the April– October growing season, it was shown that the high correlation between blocking and rainfall in southern Australia is explained by the component of rainfall associated with cut-off lows. These systems form the cyclonic components of blocking dipoles. In contrast, there is no significant correlation between the BI and rainfall from Southern Ocean fronts.

Blocking as a “driver”

Blocking serves as an important “driver” of Australian weather and climate by influencing the suppression of rainfall in the regions dominated by the anticyclonic component while contributing to enhanced rainfall in the regions where the cyclonic components are located. An association of blocking with rainfall poses a paradox because periods of blocking often accompany extended dry spells in southern Australia, including Tasmania. By way of contrast, significant rainfall events are also known to occur over southern, eastern and inland Australia when high pressure systems dominate to the south or southeast of the continent. The explanation lies in the interactive relationship between the extensive high-latitude anticyclonic component of blocking on the one hand and the cutting off or isolation of a relatively small cyclonic component equatorward of the high.

The resulting cut-off lows are known to be major contributors to rainfall events in agricultural areas of southern Australia and to runoff in Australia’s major river catchments. In blocking episodes, the Southern Ocean cyclones on the poleward side of the highs tend to be steered to higher latitudes and in exceptional cases, have been observed to cross the circumpolar trough and produce precipitation over the high plateau of Antarctica.

Blocking of atmospheric systems near the surface of the Earth occurs when a well-established poleward high pressure system lies near or within the path of the advancing storm system. The thicker the cold air mass is, the more effectively it can block an invading milder air mass. The depth of the cold air mass is normally shallower than the mountain barrier which created the cold air damming.

In the middle latitudes of the Northern Hemisphere, areas on the eastern side of blocking anticyclones or under the influence of flows from colder continental interiors related to blocks, experience severe winters, a phenomenon which has been known since the discovery of the North Atlantic Oscillation (NAO) in the 1940s. These blocking patterns also have a tendency to produce mild conditions at very high latitudes, at least in those regions exposed to anomalous flow from the ocean as in Greenland and Beringia, or from chinook winds as in Interior Alaska.

Such cold winters over the contiguous United States and southern Canada as 1911/12, 1935/36, 1977/78 and 1978/79 resulted from blocks in the Gulf of Alaska or to the east of the Mackenzie Mountains directing very cold Arctic air with a long trajectory as far as the American South, as did the Western cold waves of 1889/90 and January 1950. In Northern and Western Europe, cold winters such as 1683/84, 1739/40, 1794/95, 1829/30, 1894/95, 1916/17, 1941/42, February 1947 and 1962/63 were almost always associated with high-latitude Atlantic blocking and an equatorward shift of the polar jet stream to Portugal and even Morocco.

Over Central Asia, unusually cold winters like 1899/1900, 1929/30 and 1930/31, 1944/45, 1954/55 and 1968/69 were associated with blocking near the Ural Mountains extending the Siberian High westwards to push the very cold air from the Siberian “cold pole” outward towards the Aral and Caspian Seas. Unlike other mid-latitude regions of the Northern Hemisphere, however, cold winters in Europe (e.g. 1916/17, 1962/63) are often very mild over Central Asia, which can gain warm air advection from subtropical cyclones pushed to the south under negative NAO conditions.

Blocking Highs and other weather regimes in New Zealand

A set of 12 daily weather types for the New Zealand region has been derived from the 40-year reanalysis dataset complied by John Kidson (International Journal of Climatology, 2000).

Cluster analysis of the monthly frequencies of the patterns has led to the definition of three ‘regimes’, characterized by (i) frequent troughs crossing the country, (ii) highs to the north with strong zonal flow to the south of the New Zealand, and (iii) blocking patterns with highs more prominent in the south.

Blocking regimes are more frequent in summer and autumn and are associated with above-normal temperatures, less precipitation in the southwest of the country and more precipitation in the northeast.

The Zonal regime, which brings below-normal precipitation to the northeast and milder conditions in the south, is less common in summer.

The Trough regime is less frequent in autumn and is linked to cooler temperatures in the west and above-normal precipitation over the entire country.

The monthly frequencies of individual synoptic types are only weakly related to the Southern Oscillation Index (SOI) and other indices of the hemispheric-scale flow, with variance reductions from regression equations ranging from between 3 and 33%.

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For further Infomation about a wide range of weather/climate matters see my new book Fifteen shades of climate... the fall of the weather dice and the butterfly effect.

The book is available through the web site amazon.com. Just Google “fifteen shades of climate” for details.

www.amazon.com/dp/B08NDR1GFD?ref_=pe_3052080_397514860



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